Particles are added to many different types of compositions and products where the particles are intended to remain in particulate form after the manufacturing or finishing of the product. Product uses of particulates cover such diverse technical utilities as pigments, catalysts, colorants, fillers, matting agents, anti-slip agents, optical diffusing agents, strengthening agents, abrasion resistant agents, viscosity modifiers, reflective particles, carriers for other compounds and materials, abrasive agents, and other types of additives.
There are a number of problems that have long been associated with the use of particles in product formulations—reactivity, compatibility, dispersability, etc. The particle may be reactive and exhibit a wide range of reactivity with alkaline as well as acidic materials. In some applications particle reactivity is highly desirable (ZnO as an adhesion promoter into polymer films in paint applications) but in other applications it is most desirable to have a non-reactive particulate to prevent adverse reactions with formulation ingredients which may detract from a desired activity of a formulation component. In addition, the particle must be compatible with the formulation, to prevent particulate aggregation, and form stable dispersions, suspensions, or emulsions.
These problems are only exacerbated as particles approach nanometer dimensions by the inherently large particle surface area. The large surface area of nanoparticles must be surface treated to enable the economic advantages they impart to be realized.
Particles are also added to polymers to form bulk or thin film composite materials. The particle-matrix interface of the composite materials, as particles approach nanometer dimensions, become substantial and can strongly affect the amount of free volume in the composite material. Thus free-volume dependent properties of the composite material; such as mechanical properties, glass transition temperature, flexure strength, interaction with liquid and gas sorbants, the transport of sorbant material, dielectric and magnetic properties, etc.; may be dependant on the particle-matrix interfacial properties and controlled by the judicious selection of an appropriate surface treatment for the particulate discontinuous phase of the composite, especially as the particle approaches nanometer dimensions.
The surface treatment of particles has been addressed over the years by many different techniques and chemical efforts. Some of the techniques are the application of coatings to the surface of particles, using coupling agents on the surface of the particles, physically modifying the surface of the particles, chemically modifying the innate composition on the surface of the particles, and/or modifying the formulation to accommodate the particle—this latter is one of the least desirable methods of controlling particulate behavior in formulation as it limits formulation composition and ingredients and may alter essential formulation and product properties.
Particulate surfaces have been conventionally coated by adsorption, ion exchange, and covalent bonding. Adsorption and ion exchange require the surface to have the appropriate chemical characteristics. Reactions that enable covalent bonding to a particle surfaces generally involve reactions with a surface-bound hydroxyl group. These coatings are thin surface treatments which may afford formulation and product compatibility and for the best available technology no particulate aggregation, but can not prevent ion migration from reactive particles or affect ultimate control of interfacial material properties.